(1) Field of the Invention
The invention relates to a method of fabricating embedded memory devices, and more particularly, to a method of fabricating embedded memory devices in which one oxidation step can provide a thicker tunnel oxide and a thinner periphery transistor oxide simultaneously in the fabrication of integrated circuits.
(2) Description of the Prior Art
In the deep sub-micron process of single-poly non-volatile memory, the oxide thickness of the periphery transistors must be smaller than that of the tunnel oxide. In a conventional process, a second oxidation is performed to thicken the tunnel oxide. This composite tunnel oxide degrades the quality of the tunnel oxide. In another conventional process, a gate oxide is grown, then removed in one area. A second area is shielded by photoresist while the first area is reoxidized. Removal of the photoresist mask may cause pollution and degradation of the oxide film.
U.S. Pat. No. 5,600,164 to Ajika et al shows a memory device having three different gate oxide thicknesses, but does not describe a method for producing the differing thicknesses. U.S. Pat. Nos. 5,498,577 to Fulford, Jr. et al and 5,591,681 to Wristers et al teach methods involving two oxidation steps in which a portion of the first oxide is removed and then the second oxidation forms a second thinner oxide and thickens the first oxide film. U.S. Pat. No. 5,502,009 to Lin teaches a method in which a portion of the first oxide is removed and regrown as a thicker oxide layer. A silicon nitride shield over the non-removed first oxide film prevents further growth of the first film. U.S. Pat. No. 5,668,035 to Fang et al teaches a similar method to Lin in which a thinner second oxide layer is regrown while the first oxide film is shielded from oxidation by a polysilicon layer. U.S. Pat. No. 5,480,828 to Hsu et al discloses a method of implanting nitrogen ions into a portion of a silicon substrate. The presence of the nitrogen ions slows the oxidation rate over that area of the substrate resulting in a thinner gate oxide film in that area using one oxidation step. However, the nitrogen implantation is not a good idea for tunnel oxide because the implant will damage the substrate.